1. Field of the Invention
This invention relates to an improved process for etching tungsten on a semiconductor wafer. More particularly, this invention relates to an improved process for etching a layer of tungsten having a patterned layer of photoresist wherein the etch process is very selective to photoresist and exhibits low critical dimension loss.
2. Description of the Related Art
In the formation of tungsten lines in an integrated circuit structure formed on a semiconductor wafer, it is important to provide an etch process which is highly selective to the patterned mask layer, i.e., will preferentially etch the exposed portions of the underlying tungsten layer, not the mask layer.
This has become increasingly important in the formation of VLSI structures where both the line width and the pitch (line width plus spacing between lines) of the structure is very small, i.e., about 0.6 micron line width and 1.5 micron pitch. If the mask material, such as a patterned photoresist layer, is also etched during the etching process, in particular, if the sidewalls of the mask openings are etched, the resulting tungsten line width will be unacceptably smaller than the original mask width, sometimes referred to as the critical dimension.
This is shown in FIG. 1 wherein portions 10 of a photoresist pattern are shown over tungsten lines 20 formed (by a prior art process) on a semiconductor wafer 30 containing other portions of a integrated circuit structure. The "critical dimension" shown in the FIGURE indicates the critical line width dimension which should have been etched in the tungsten layer. Dotted lines 12 (exaggerated for illustrative purposes) indicate how much the lateral dimensions of portions 10 of the photoresist pattern were shrunk or eroded by the etch resulting in a narrower line width, i.e., smaller critical dimension, of tungsten lines 20 in comparison to dotted lines 22 which indicate what the desired line width of the tungsten lines should have been.
The critical dimension of line width of the tungsten line may also be affected by an undercutting or erosion of the width of the tungsten line. This problem is depicted in FIG. 2 wherein tungsten lines 24 on wafer 30 are indicative of how the sidewalls of the openings in the tungsten layer may be laterally etched, even when photoresist mask portions 10' were not laterally etched, resulting in an unacceptable narrowing of the critical dimension of line width of the tungsten line being formed.
It would, therefore, be desirable to provide a process for etching a tungsten layer through a photoresist mask layer wherein the critical dimension of the resulting tungsten lines would not be materially reduced due to either etching of the sidewalls of the openings in the photoresist mask or to undercut erosion of the tungsten layer by lateral etching of the tungsten beneath the photoresist mask.